A lighting circuit for a discharge lamp, such as a metal halide lamp, is known which incorporates a DC power source, a switching power source circuit, a DC-AC converting circuit and a start circuit. As a control method for a switching power source circuit formed into a DC--DC converter circuit, a PWM (Pulse Width Modulation) method is known. The method is structured such that the frequency (a so-called switching frequency) of a control signal for a switching device in a converter circuit is made to be constant. Moreover, the ON/OFF ratio (duty ratio or duty cycle) is varied so that the output voltage is changed. For example, control is performed such that when the ON period of the switching device is elongated, the OFF period can be shortened.
When the conventional lighting circuit encounters a nonuniform OFF period of the switching device with respect to change in the switching frequency, there arises a problem in that the efficiency of using the power source deteriorates owing to change in the power supply voltage of the lighting circuit. Another problem arises in that the stability of lighting of the discharge lamp cannot easily be maintained with respect to change in the status (lowering of the tube voltage) of the discharge lamp (for example, the discharge lamp is undesirably turned off).